This invention relates to tube couplings of the kind having a collet comprising a cylindrical portion with a cylindrical bore and with resilient arms extending axially from one end of said cylindrical portion and arranged to cooperate with a cam surface. The cam surface is constituted by a member surrounding the collet and having an internal surface tapering to a smaller diameter in one axial direction and engaging the arms so that the arms are forced radially inwardly by axial movement of the collet in said one direction to engage and lock a tube in the coupling. The collet may have a head at its outer end to facilitate locking and releasing of a tube in the coupling. It will be understood that operation of the collet head, particularly in the case of very small couplings for small diameter tubes in confined spaces, can cause difficulties and it is an object of the present invention to facilitate release of the collet when required.